This invention relates generally to vehicle mounted aerial devices and more particularly to a mechanism which acts to articulate the upper boom of an articulating boom assembly.
Vehicle mounted aerial devices have long been used for a variety of applications such as performing work on utility poles, trimming trees, maintaining street lights, and servicing overhead power and telephone lines. The aerial device normally includes a multiple section boom which can either be an articulating boom or a boom that is extensible and retractable in telescoping fashion. The tip of the upper boom is equipped with a personnel carrying device which is typically a bucket or basket. More than one bucket or basket may be carried on the boom, and the bucket or basket may be large enough to carry more than one worker. Supplemental load lifting devices may also be installed on the boom near the platform in order to provide the aerial device with lifting capabilities. The load lifting device is typically an adjustable jib, a winch, or a combination of both.
Although both the articulating and telescoping booms have advantages, the ability of the articulating aerial device to vary the angle between the upper and lower booms makes it generally more versatile in that it permits workers to reach more areas without the need to move the vehicle or place it in unusual positions such as on sidewalks, in fields, or in roadside ditches. By way of example, workers can be given easy access to the field side of a utility pole by extending the lower boom underneath the power or telephone lines and raising the upper boom until the platform is adjacent to the field side of the pole. In an articulating boom, it is desirable for the upper boom to have as large an arc of articulation as possible and to have the ability to lift the workers and supplemental loads throughout its entire range of articulation. The ability of the upper boom go "overcenter" or past the vertical position is an important attribute in many applications.
At present, there are two types of mechanisms that are in general use to articulate the upper boom of an aerial device. The first type is referred to in the industry as a four bar or scissors linkage. The other type is known as a constant radius mechanism.
The four bar linkage is relatively easy to maintain and is capable of handling large loads through most of its operating range. However, detracting from these advantages are several disadvantages. The moment or torque applied to the load varies rather widely as the angle of articulation changes, and the angular speed is likewise variable with the articulation angle. Near the ends of its travel, the upper boom is moved relatively fast and has a small lifting capacity. Conversely, the upper boom can handle heavy loads but moves slowly near the center of its arc. Four bar linkages are also somewhat limited in the arc through which the upper boom can be moved. If more than about 210.degree.-225.degree. of articulation is needed, four bar linkages are impractical. A final problem is that any looseness in the pivot connections results in play in the linkage when the machine is moved overcenter and the forces applied to the linkage components are suddenly reversed. This causes undesirable jerkiness in the boom operation and applies abrupt shock loads on the components of the four bar linkage.
The constant radius mechanism normally includes a pulley or sprocket on the upper boom and a cable or chain which is drawn around the pulley or sprocket and operated by one or more hydraulic cylinders. In this type of arrangement, there is no change in the angular velocity as the angle of articulation changes because the moment which is applied to the pulley or sprocket is constant. For the same reason, the lifting ability of the machine closely parallels the structural capacity of the boom. Additional advantages are the relatively large angles of articulation that can be achieved (220.degree.-270.degree.) and smoother operation when the machine moves past the overcenter position. However, when a chain and sprocket mechanism is used, there is inevitably some looseness between the sprocket teeth and the links of the chain, especially when they are worn. Thus, at the overcenter position, the sprocket teeth reverse and load the opposite ends of the chain links. Noticeable jerkiness in the boom operation can result, and the wear is greatly accelerated.
Further disadvantages associated with the constant radius mechanism include higher cost of the components and limited lifting capacity caused by the lack of availability of components large enough to provide the necessary strength. Even more importantly, chain and sprocket and cable and pulley devices are notoriously susceptible to wear which can make them dangerous unless they are frequently inspected and regularly serviced and/or replaced. The costs resulting from the high maintenance requirements can be substantial.